Significant recent increase in g++ fails on cygwin

[DJ Delorie]

> However, all the EH tests seem to fail for me on a freshly built cygwin
> toolchain.  There seems to be a problem with registering the dwarf2 unwind
> information.  Are you configuring with --enable-sjlj-exceptions?

There is (or at least was) a bug in cygwin (not gcc) in that gcc's
crtstuff.c isn't used, so dwarf2 info can't be registered.  SJLJ
exceptions are required.